Does anyone happen to have information on the predatory behaviour of Crossocerus megacephalus? I ask because a few years ago I found a nest of this species in the SE of England and upon investigation it was found to contain three brood chambers. The larvae in two of these chambers had completed their development; however the third was still sealed and inside there were 15 Chrysopilus asiliformis.

Is this wasp a specialist on rhagionids or do you think what I found was a consequence of a local abundance of this fly species?

I had a quick look into Blösch - Grabwespen Deutschlands (2000). He summarized much of the literature and mentions for this species that its preying many different kind of flies from very different families even and in different sizes. So i guess ur finding is not unusual.